During working of an undersea station having a collecting head or wellhead, the production riser is normally suspended from a floating structure, for example a semi submersible structure possibly having dynamic positioning means.
In order to lighten the weight of a central tube recourse is had to floatation devices, for example by surrounding the central tube with half shells made from a material with positive buoyancy held by straps. In this case, the installation of the peripheral stringers as an external stack is provided by means of steel guide funnels carried by the central tube. Such arrangements make the assembly very heavy and not easy to handle.